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Cowling Plug – Rapid Prototyping

This is a project that I have been working on as part of my Computer Aided Drafting education.  It is a rather big project as it involves a wide variety of skill sets.  Below I’ll explain the overall process.

The current prototype airplane I am building is going to require an engine cowling and spinner for the propeller.  I was able to use my new Autodesk Inventor (ETEC 270) skills to loft both surfaces in 3D based on the limiting size factors of the airframe and engine.  Then it was just a matter of designing a pleasing shape.

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The Fab Lab at my local community college (Metropolitan Community College) has a Techno CNC Router Table that is rather amazing.  Not only is it huge, but very accurate as well.  This would be my first time operating a machine like this, so I had a steep learning curve.  When you start to think about your CAD model being fabricated, often the part gets limited by the machine abilities.  For me, typically this is the Z-axis travel.  Here is a picture of the machine in all of its glory.

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With the high repeat-ability of the machine, sectioning your model into slices is now possible.  This really opened my mind to endless possibilities.  Anyway, back to the part.

With help from the Fab Lab staff we were able to source a new square end mill with 3 inches of cutting surface for the CNC router.  This longer bit allowed me to improve the Z axis cutting depth by a factor of 4.  In order to save machine time and programming time, I sectioned the solid model into 4″ slices.  These 4″ thick blocks could be cut from a single tooling path using a 2″ step on the Z axis.

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The first tooling path we generated had an 8 hour run time.  Quickly we decided that a new software would be very helpful.  The school was in the process of getting Aspire CNC software already.  After figuring out how to use it, I cut that 8 hour run time to under 2 hours average per layer by optimizing the tooling path!  Not to brag, but that is sort of a big deal. 🙂

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I purchased $350 worth of polypropylene foam from Home Depot in 4’x8′ sheets, 2″ thick.  I couldn’t find anything thicker locally so I laminated 2 sheets together to make 4″ thick sheets to match the 3D profiles. These sheets were stuck to the CNC table using double sided carpet tape and we let her rip!

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It has taken a few weeks to complete the entire profile because I’m not the only one that knows the value of this machine.  Regardless, the end result I am very happy with.  It would have been nearly impossible to hand sand a compound curve surface and get it symmetrical. 

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This foam will be used as a plug to manufacture a high temperature mold.  The mold will then be used to make the final part, a fiberglass cowling with nomex honeycomb core where needed.